For what it is worth I use both, in much the same way as you do, and I haven't been arrested or charged yet.

I really don't think that there is any body with the power or authority to give or withhold permission. I suggest that you do what you think is appropriate. If people really don't like it they'll let you know, and you can decide to change or not depending on whether their reasons make sense.

Another question about the en/emdashes... does it matter if they're orphaned and fall onto the next line? I know I avoid it for ellipses, but it's never bothered me if an emdash should fall on the next line like so (but I could be very wrong about that since I'm still such a newb at this and perhaps it should be avoided too):

Yes: except for a brief period, the house had been untenanted for some time
—for fifteen years, in fact.

Another question about the en/emdashes... does it matter if they're orphaned and fall onto the next line? I know I avoid it for ellipses, but it's never bothered me if an emdash should fall on the next line like so (but I could be very wrong about that since I'm still such a newb at this and perhaps it should be avoided too):

Yes: except for a brief period, the house had been untenanted for some time
—for fifteen years, in fact.

I don't like an em-dash on the next line.

If we could rely on renderers, I would suggest that for
<word><space><en-dash><space<word>
you use
<word><no-break space U+00A0><en-dash><space><word>.

For
<word><em-dash><word>
you could use
<word><word joiner U+2060><em-dash><word>

But most renderers will do what they want anyway, and might display an unknown character symbol.

The book I'm doing has no spaces before and after the em dashes so I want to keep it that way. But I can't find a way to prevent the orphans that works consistently. If I use & # 8288 ; (without the spaces, or it wouldn't display)⁠ before the em dash in Sigil for instance, Sigil's viewer displays what looks like a thick pipe character. Calibre's viewer shows a space. When loaded on my Sony 350 though, it looks perfect... no space, no odd character, and no orphan em dashes.

Is it safe to assume it will work on all ereaders correctly then as it does on the Sony or might it depend on the font used? Or is there a different code to use in Sigil so that it will display correctly no matter where viewed, cell phones, PC, Sigil, Calibre, etc?

You would think that software across all platforms could follow certain rules to avoid orphans of certain characters.

In Kindle for PC, the mobi shows a square box like a typical missing character before each em dash. When I check it with Kindle Previewer though, it looks perfect in all 3 choices whether Paperwhite, Kindle or KindleDX.

I'm not sure I should risk the odd boxes or spacing issues for some people now, or if it's better just to let the orphans stay as they were.

Edit: The above results were converting as "old" mobi type, which I usually prefer to keep the file size down. I then converted it as "both" in the Calibre options and now in the Kindle Previewer, the Paperwhite view shows a space before each em dash. All else looks the same as in the "old" mobi format. I really didn't expect to see a difference there, but glad I tested it.

Speaking as an editor and copyeditor, you do want to be consistent. The em dash and en dash have specific uses, and a professionally edited book would never conflate them. The em dash is used for any break in thought, so the two examples you gave would both take em dashes. You wouldn't use two em dashes at the end of a sentence—it's for situations such as to indicate missing l——rs in a word. A three-em dash is used for a missing ——— in a sentence or the same author as the previous citation in a bibliography.

The en dash is used between a range of numbers (e.g., 25–30) or months (a "May–December marriage") to clarify that it's not intended as a hyphenated term. It's also used where a prefix modifies both terms of a compound expression, and hyphenation would be misleading—for example, a "pre–World War II" building (you wouldn't hyphenate proper nouns: "pre-World-War-II") or a "New York–Hong Kong flight" (you wouldn't say "New-York-Hong-Kong.") (In newspapers, you often see a hyphen used for this purpose, because journalists on daily deadlines don't have time for such niceties.)

The authority for these questions is The Chicago Manual of Style, which is the standard reference in book publishing. Chapter 5 covers punctuation. They also have an online version, to which you can subscribe.

As far as an em dash breaking to the next line, that's not considered a problem; in fact, it's often inevitable. In books, em dashes are closed up to the text on either side. On the web, browsers see one long word, consisting of the word on either side and the dash in between, so dashes on the web are usually open, with a space between them and the surrounding words. An ebook will behave the same as a web page, so you may get some awkward breaks, but what are the chances that two words with an em dash between them will fall at the end of a line, especially since readers can change the type size? In my books, I made the decision to keep the dashes closed up, because it's more like a print book.